Modern medical imaging methods normally provide images in digital form. To this end, the first step within the framework of “primary applications” is data recording and the provision of the digital data in the course of data construction. In particular, computed tomography images are provided in digital form and can thus be processed further directly in a computer or in a workstation. From the original images, it is possible to obtain images in a new orientation with two-dimensional or three-dimensional display (2D display, 3D display) in order to provide a suitable overview for the examiner. Such displays are intended, in particular, to form the basis of subsequent diagnosis within the context of a monitor examination.
Advantages of computed tomography result, in particular, from the fact that there are no superposition problems as in the case of conventional radiography. Further, computed tomography provides the advantage of undistorted display regardless of different magnification factors associated with the recording geometry in radiography.
In the meantime, a series of different procedures have become established for 3D image display and processing. For these procedures, a computed tomograph has suitable control elements, e.g. a computer mouse or other control media. A workstation for image display and processing of computed tomography images is equipped with appropriate software in the form of a computer program product and a user interface on a screen with appropriate control elements to which functions are assigned.
Computed tomography (CT) first of all normally provides two-dimensional sectional images of the transverse plane of a body to be examined as direct recording plane. In this case, the transverse plane of a body is arranged essentially at right angles to the longitudinal axis of a body. Two-dimensional sectional images in a plane at an angle that has changed in comparison with the transverse plane and/or those which are calculated with a different, particularly broader, layer thickness than the original layer thickness are normally called multiplanar reformations (MPR).
One option which is fundamental to diagnosis is interactive inspection and evaluation of the image volume, usually under the control of an appropriate control element. The examiner can use such control elements—in a similar manner to guiding a sound head in ultrasound—to feel his way to anatomized structures and pathological details and can move forward and backward to select that image in which a detail of interest is presented most clearly, that is to say by way of example is displayed with the highest contrast and the largest diameter.
An extended form of two-dimensional display involves putting together layers (slabs) of arbitrary thickness from thin layers. For this, the term “sliding thin slab” (STS) has become established.
All 2D displays have the advantage that the computed tomography values are displayed directly and without corruption. Any interpolations or averages formed over a plurality of layers are negligible in this case. Thus, there is always simple orientation in the evaluation volume, which is also called the volume of interest (VOI), and in the associated 3D data volume and also explicit interpretability of the image values. This type of monitor examination is work-intensive and time-consuming, however.
By contrast, the most realistic presentation of the evaluation volume possible can be achieved through three-dimensional display of the evaluation volume. Although 3D image display and processing is normally the prerequisite for specific elaboration of diagnostically relevant details, the latter examination is normally performed in a 2D display.
In the case of 3D image display and processing, a 3D data volume is normally provided which is taken as a basis for displaying the evaluation volume. The examiner preferably prescribes an observer position from which he wishes to observe the evaluation volume. In particular, the examiner normally has a search beam at his disposal. In this example, a two-dimensional image is calculated which is at right angles to the search beam and is intended to convey a spatial impression.
To construct such a display pixel by pixel (also: voxel—acronym for volume element) in the image plane, all CT values along the search beam through the 3D data volume need to be taken into account and assessed for each beam from the observer to the respective pixel. The examiner normally prescribes a pixel value, e.g. a contrast value, which he selects in suitable fashion for displaying a pixel. The repetition (inherent to the method) of this process shows the examiner a collection of pixels corresponding to the search beam on the basis of the prescribed pixel values within the context of a CT value profile for the search beam, that is to say shows a 3D display of the body region/evaluation volume of interest (VOI).
All 3D displays may, that is to say within the context of a secondary application, be designed either as a central projection or as a parallel projection. For a parallel projection, “maximum intensity projection” (MIP) or generally “volume rendering” (VR) is particularly suitable.
In the case of MIP, the pixel with the highest CT value is determined in the projection direction along the search beam. In that case, the pixel value thus corresponds to the maximum CT value on the search beam.
In the case of VR, not just a single pixel is chosen for each individual search beam coming from the observer's eye. Rather, all CT values along the search beam can, with suitable weighting, deliver a pixel as a contribution to the resulting image.
Freely selectable and interactively alterable transfer functions are used to assign opacity and color to each pixel value. It is thus possible, by way of example, to select normal soft tissue to be largely transparent, contrasted vessels to be slightly opaque and bones to be very opaque. Preferable central projections may be attained, by way of example, by “surface shaded display” (SSD) or by “perspective volume rendering” (pVR) (or else “virtual endoscopy”). Accordingly, there is the SSD or else the pSSD used in virtual endoscopy.
SSD is threshold-based surface display, where a pixel is prescribed by prescribing a pixel value in the form of a threshold. For every search beam through the present 3D data volume, that pixel is determined at which the prescribed pixel value in the form of a threshold value is reached or exceeded for the first time as seen by the observer.
One basic difference between SSD and VR is that in the case of SSD only one threshold is defined, but the surface is displayed opaque. In the case of VR, on the other hand, a plurality of threshold regions are defined and these are assigned colors and transparencies.
“Virtual endoscopy” is intended to permit a perspective view of the close surroundings of the virtual “endoscope head”. Unlike in the case of the actual endoscope, structures can be observed from different directions and while moving. “Fly throughs”, which are intended to give the impression of a virtual flight through the VOI, are possible. This is not only esthetic and instructive, but also may be of diagnostic value. In particular, a “vessel view” method can be used to render the interior of an evaluation volume visible.
For a parallel projection, a “maximum intensity projection” (MIP) or generally the “volume rendering” (VR) is particularly suitable. In the case of an MIP, the pixel with the highest CT value is determined in the projection direction along the search beam. In that case, the pixel value thus corresponds to the maximum CT value on the search beam.
In the case of the VR, not just a single pixel is chosen for each individual search beam coming from the observer's eye, but rather all CT values along the search beam can, with suitable weighting, deliver a pixel as a contribution to the resulting image. Freely selectable and interactively alterable transfer functions are used to assign opacity and color to each pixel value. It is thus possible, by way of example, to select normal soft tissue to be largely transparent, contrasted vessels to be slightly opaque and bones to be very opaque.
A prerequisite for image display in virtual endoscopy is normally a statement relating to an observer path. Such an observer path is also called a flight path or a center line. The observer path corresponds in practice to the path which is taken by a virtual endoscope head and along which a perspective view of the close surroundings is reproduced. In this case, the problem frequently arises that a body part to be examined in the VOI is split into a plurality of evaluation volumes.
In that case, there is no continuous observer path through the VOI. Rather, it is necessary to provide an observer path in the first evaluation volume and then to change to a second evaluation volume, with an observer path then needing to be provided again there. Such a situation may arise, by way of example, when a tubular formation in the VOI, for example a colon, a cisterna or a bronchial system, has a restriction or a closure or is interrupted in another way, for example is not filled with contrast agent, or other circumstances lead to the tubular body part on which the endoscope is to be used not being in the form of a single complete evaluation volume.
In this regard, a user of the usual method who is examining a patient normally needs to find the respective first and second evaluation volume and to define the conditions, e.g. a starting point, for an observer path from the very beginning. In that case, the examiner needs to use a large part of his time to find all evaluation volumes in order to examine the entire VOI. A new observer path needs to be provided in every single evaluation volume. This costs time and results in the “fly through” needing to be interrupted or even parts of the VOI not being examined.
It would be desirable for a virtual endoscopy to involve a VOI being examined as reliably and completely as possible.